Art & Artifacts is the book they released earlier this year which was packed with amazing artwork and illustrations from every game in the series.

Hyrule Encylopedia's first chapter will contain a visual guide to key terms in the series ("Master Sword", "Hyrule Castle", etc.). Its second chapter will be a database of enemies, items, dungeons, and more. The third chapter will contain an "archives" and will contain notes about the storylines of each game. I'm hoping the "archives" is full of development material.

Soku and I picked up a copy of the newly released Hyrule Encyclopedia (sequel to Hyrule Graphics/Zelda Art & Artifacts), and it had a lot of great design documents and development stuff in it, including this screenshot:

This is the only screenshot known to exist from this super early build of the original Zelda. This comes from back when the game had no overworld at all and was only dungeons. This screen is the dungeon select menu, which was discussed in more detail in this old Iwata Asks interview.